Australasian Plant Conservation
Originally published in Australasian Plant Conservation 17(3) December 2008 – February 2009, p 8-10
Experimental approaches in threatened
plant translocations: how failures can
still lead to success
Department of Environment and Conservation, Kensington, WA. Email: firstname.lastname@example.org
Figure 1. Translocated Lambertia orbifolia subsp. orbifolia seedlings, between 1998 and 2006: percent survival (A), mean height (m) (B) and mean crown width (m) (C) of seedlings with (dark bars) and without (white bars) tree guards.
Figure 2. Translocated Banksia ionthocarpa subsp. ionthocarpa seedlings, between 1999 and 2007: percent survival (A), mean height (m) (B) and mean crown width (m) (C)
of seedlings with (dark bars) and without (white bars) shade.
Figure 3. Translocated Daviesia bursarioides seedlings, between 1999 and 2008: percent survival (A), mean height (m) (B) and mean crown width (m) (C) of seedlings with (dark bars) and without (white bars) mulch.
Translocations are increasingly used to aid in the recovery of threatened plant species. This is achieved by augmenting declining populations and supporting species recovery through the establishment of new populations. Translocations are often thought of as a quick fix solution to prevent species or population extinction. However, translocations require detailed planning prior to establishment, followed by long-term monitoring and maintenance. Without this they are likely to fail.
The aim of plant translocations should be to create viable self-sustaining populations (biological success). However, broader success criteria should also be considered (project success). These project success goals should not only encompass biological success but include additional goals, such as learning more about the translocation process and communicating these results (successes and failures) to the wider conservation community to allow others to gain from the experience (Pavlik 1996).
In Western Australia plant translocations have been used in the recovery of threatened plant species since the early 1990s. In most cases the fate of the plants from these early translocations were not adequately monitored. Consequently, little knowledge was gained that could improve the success of subsequent translocations.
Since 1997 the Department of Environment and Conservation has commenced translocations for 44 threatened plant species and ensured these were established in a way that knowledge could be gained from any successes or failures. In most cases the translocations were established as experiments with the aim of creating viable populations, and refining translocation techniques. In order to achieve the latter aim, a range of practices commonly used in horticulture were investigated for their potential to enhance the survival and growth of the translocated plants in a cost-effective manner. The results from experimental trials of three common treatments are reported here.
One of the techniques trialed was the use of plastic tree guards, which are commonly used in roadside and restoration plantings to protect plants from frost, wind and herbivores. They are also thought to enhance growth by elevating humidity and concentrating carbon dioxide. To test whether the use of tree guards improved survival or enhanced growth of translocated seedlings, a trial with the critically endangered Lambertia orbifolia subsp. orbifolia (Round Leaf Honeysuckle) was established in 1998.
At a translocation site located north of Albany in south-west Western Australia, 108 seedlings were planted. Half the seedlings were given tree guards (approximately 0.5 m in height and 0.3 m in diameter) and half were fenced with cages made from wire netting (0.9 m in height and 0.5 m in diameter). Fencing the plants without tree guards meant comparisons of the effectiveness of tree guards in preventing herbivory could not be made, but other benefits could still be assessed. Annual assessments of survival, height and crown width were undertaken. However, as the plants grew taller than the tree guards by the end of the second year, it was considered that the main effects of the tree guards would be seen in the first two years.
At the conclusion of the first and second years there was no difference in survival between the plants with and without tree guards (P = 0.35, P = <0.01) (Fig. 1A). Plants with tree guards grew taller than plants without tree guards in the first year (P = 0.01), but by the second year this height difference was no longer significant (P = 0.47) (Fig. 1B). Conversely, plants with tree guards had narrower canopies in the first year than plants without tree guards
(P = 0.28) but this had reversed by the second year
(P = 0.26) (Fig. 1C).
It appears that horizontal canopy growth is constrained by the tree guard and therefore plants grow vertically more rapidly than plants without them. Once taller than the tree guards, horizontal canopy growth can occur. The site location may also have played a role in the effectiveness of the tree guards. Located amongst shrubs, with a woodland overstorey, frost and wind are unlikely to have had a significant impact on plant survival or growth.
It was hypothesized that translocated seedlings provided with shade would survive better and have greater growth than those seedlings without shade, due to less transpiration. We implemented this technique when translocating the critically endangered Banksia (formerly Dryandra) ionthocarpa subsp. ionthocarpa. We planted 95 seedlings in winter 1999 at a translocation site north of Albany. After planting, each plant was enclosed in a circular cage made of wire netting (0.5 m in diameter), which was open at the top. Half the seedlings had the cages covered in 80% block-out green shade cloth (shaded treatment) and half were left unshaded (control).
After eight years, overall survival was very low, with just four seedlings surviving in the shaded treatment and two control seedlings (Fig. 2A). The difference in survival was not significant (P = 0.57). Shaded plants were slightly taller on average (0.28 m) compared to the control plants (0.22 m) but this was not significant (P = 0.67) (Fig. 2B). In contrast, control plants were slightly wider (0.26 m) compared to the shaded plants (0.22 m), but again this was not significant (P = 0.07) (Fig. 2C). Overall, shading did not significantly enhance survival or growth, and survival of translocated plants was poor in general.
The use of mulch is widely encouraged in garden and horticultural plantings as it is thought to aid water retention and help reduce soil temperature around the root zone. We tested whether mulch could enhance the survival and growth of translocated seedlings when augmenting a small population of the critically endangered Daviesia bursarioides (Three Springs Daviesia). We planted 96 seedlings into a nature reserve 300 km north of Perth in August 1998. Half of the seedlings had sterilized mulch (made from ground-up tree loppings) spread around their base in a 1 m diameter area and thickness of 2-3 cm; the remaining seedlings had no treatment. Plants were monitored for growth and survival annually or biannually.
The use of mulch did not improve plant survival, with only 10% of plants in both treatments persisting after ten years (Fig. 3A). Addition of mulch also did not result in greater growth compared to plants with no treatment (P = 0.37 for height and P = 0.49 for crown width) (Figs 3B and 3C).
None of the three translocations discussed could be considered successful in a biological sense. In all cases plant survival was low and the resulting small populations are unlikely to be viable in the long term. However, with failure also came success.
The results from the tree guard trial suggest that the guards did not work in that situation, and consequently tree guards have not been used in subsequent, more successful plantings of Lambertia orbifolia subsp. orbifolia. The shade trial revealed time and resources would be wasted, if adopted on a large scale for Banksia ionthocarpa subsp. ionthocarpa, as this technique was not successful in decreasing the taxon’s plant mortality or increasing plant growth. The mulch trial with Daviesia bursarioides showed that the added expense and time for this treatment had no apparent biological advantage, although the trial was successful in that the results have aided subsequent translocation efforts by reducing costs and staff time.
The above results demonstrate the importance of small-scale trials of new techniques aimed at improving plant survival, rather than assuming what works in other plant industries will also work for the species and environments we are required to manage. The design and establishment of translocations and subsequent collection and dissemination of knowledge about what techniques do, or do not, improve translocation success is critical to achieving optimal results in a cost-effective manner. This ensures broader translocation criteria are met and increases the likelihood of creating viable self-sustaining plant populations.
Pavlik B.M. (1996). Defining and measuring success. In: D.A.Falk, C.I.Millar and M.Olwell (eds). Restoring Diversity. Strategies for Reintroduction of Endangered Plants, pp. 127-55. Island Press, Washington DC, USA.